The root differs from the shoot in the characters of its surface tissues, in the absence of the green assimilative pigment chlorophyll, in the arrangement of its vascular system and in the mode of growth at the apex, all features which are in direct relation to its normally subterranean life and its fixative and absorptive functions.
The vascular tissue is typically separable into distinct collateral bundles (figs.
In such cases the vascular system is said to be polycyclic in contrast with the ordinary monocyclic condition, These internal strands or cylinders are to be regarded as peculiar types of elaboration of the stele, and probably act as reservoirs for water-storage which can be drawn upon when the water supply from the root is deficient.
This is especially the case in the young vascular bundles themselves (desmogen strands).
In the Vascular Plants this tissue is collectively known as the vascular system.
In higher forms the conducting strands of the leaves are continued downwards into the stem, and eventually come into connection with the central hydrom cylinder, forming a complete cylindrical investment apparently distinct from the latter, and exhibiting a differentiation into hydrom, leptom and amylom which almost completely parallels that found among the true vascular plants.
It is essentially a living tissue, and serves to place all the living cells of the secondary vascular tissues in communication.
In one type they may take the form of specially-modified single epidermal cells or multicellular hairs without any direct connection with the vascular system.
Such a vascular cylinder is called a haplostele, and the axis containing it is said to be haplostelic. In the stele of the root the strands of tracheids along the lines where the xylem touches the pericycle are spiral or annular, and are the xylem elements first formed when the cylinder is developing.
The tissue-elements just described are found only in the more complicated secondary vascular tissues of certain Dicotyledons.
The body of a vascular plant is developed in the first place by repeated division of the fertilized egg and the growth of Develop- the products of division.
But these two principles do not find their full expression till we come, in the ascending series, to the Vascular Plants.
The surface layer of the rhizome bears rhizoids, and its whole structure strikingly resembles that of the typical root of a vascular plant.
The stelar system of Vascular Plants has no direct phylogenetic connection with that of the mosses.
The evolution of the vascular structure of the petiole in the higher ferns is strikingly parallel with that of the stem, except in some few special cases.
I&Vertical section of a Palm-stem showing the root as it is found in most Pteridophytes vascular bundle,, Jr. curving and many Phanerogams has been already inwards and then outwards.
The radial structure is characteristic of all root-steles, which have in essential points a remarkably uniform structure throughout the vascular plants, a fact no doubt largely dependent on the very uniform conditions under which they live.
In many Pteridophytes thi first leaf is formed very early, and the first vascular strand i!
In the very frequent cases where the bundles have considerable individuality, the fibrous pericyclic cap very clearly has a common origin from the same strand of tissue as the vascular elements themselves.
The vascular supply of the leaf (leaf-trace) consists of a single strand only in the haplostelic and some of the more primitive siphonostelic forms. In the microphyllous groups Leaf.trace of Pteridophytes (Lycopodiales and Equisetales) in and Petlolar which the leaves are small relatively to the stem, the Strands, single bundle destined for each leaf is a small strand whose departure causes no disturbance in the cauline stele.
There is good reason to believe that the haplostele is primitive in the evolution of the vascular system.
The typical structure of the vascular cylinder of the adult primary stem in the Gyrnnosperms and Dicotyledons is, like that of the higher ferns, a hollow cylinder of vas- Structure of cular tissue enclosing a central parenchymatous pith.
In the stems of many water-plants various stages of reduction of the vascular system, especially of the xylem, are met with, and very often this reduction leads to the formation of a compact stele in which the individuality of the separate Reduced bundles may be suppressed, so that a closed cylinder lmpbost~h1c of xylem surrounds a pith.
In the blade of a typical leaf of a vascular plantessentially a thin plate of assimilating tissuethe vascular system takes the form of a number of separate, usually branching and anastomosing strands.
Later, the axis branches by the formation of new growing-points, and in this way the complex system of axes forming the body of the ordinary vascular plant is built up. In the flowering plants the embryo, after developing up to a certain point, stopf growing and rests, enclosed within the seed.
In this case also the differentiation of leaf-bundles, which typically begins at the base of the leaf and extends upwards into the leaf and downwards into the stem, is the first phenomenon in the development of vascular tissue, and is seen at a higher level than the formation of a stele.
Where a large-celled pith is developed this often becomes obvious very early, and in some cases it appears to have separate initials situated below those of the hollow vascular cylinder.
The differentiation of the stelar stereom, which usually takes the form of a sclerized pericycle, and may extend to the endocycle and parts of the rays, takes place in most cases later than the formation of the primary vascular strand.
The periblem, one cell thick at the apex, produces the cortex, to which the piliferous layer belongs in Monocotyledons; and the plerome, which is nearly always sharply separated from the periblem, gives rise to the vascular cylinder.
Thi vascular system is connected in various ways with that of th(parent axis by the differentiation of bundle-connections across thi cortex of the latter.
Every great group or phylum of vascular plants, when it has become dominant in the vegetation of the world, has produced members with the tree habit arising by the formation of a thick woody trunk, in most cases by the activity of a cambium.
The main feature is the development of special vascular stereom and storage tissue.
The viscid pulp soon hardens, affording a protection to the seed; in germination the sucker-root penetrates the bark, and a connexion is established with the vascular tissue of the first plant.
This observation led him to further work, and he succeeded in showing that in vascular organs the presence of cells in inflammatory exudates is not the result of exudation but of multiplication of pre-existing cells.
The sporophyte is the plant which is differentiated into stem, leaf and root, which show a wonderful variety 01 form; the internal structure also shows increased complexity and variety as compared with the other group of vascular plants, the Pteridophyta.
In many forms its hyphae are particularly thick-walled, and may strikingly resemble the epidermis of a vascular plant.
Outside this are three arcs of large cells showing characters typical of the endodermis in a vascular plan.t; these are interrupted by strands ofnarrow, elongated, thick-walled cells, which send branches into the little brown scales borne by the rhizome.
The body of the sporophyte in the great majority of the vascular plants shows a considerable increase in complexity over that found in the gametophyte of Bryophytes.
In the Vascular Plants this tissue is collectively known as the vascular system.
The structure of the stomata of the sporophyte of vascular plants is fundamentally the same as that of the stomata on the sporogonium of the true mosses and of the liverwort A nihoceros.
One of the most striking characters common to the two highest groups of plants, the Pteridophytes and Phanerogams, is the Vascular possession of a double (hydrom-leptom) conducting .s system, such as we saw among the highest mosses, YS em.
It is confined to the sporophyte, which forms the, leafy plant in these groups, and is known as the vascular system.
C eun~7 Such an arrangement of vascular tissue is called radial, ~
Ft1.Types of Stole in Vascular Plants.
The type of siphonostele characteristic of many ferns, in which are found internal phloem, and an internal endodermis separating the vascular conjunctive from the pith is known as a solenostele.
The splitting up of the vascular tube I into separate strands does not depend wholly upon the occurrence I of leaf-gaps.
In some forms other gaps (perforations) appear in the vascular tube placing the pith and cortex in communication.
In other cases the leaf-gaps are very broad and long, the meristeles separating them being reduced to comparatively slender strands, while there is present in each gap a network of fine vascular threads, some of which run out to the leaf, while others form cross-connections between these leaf-trace strands and also with the main cauline meristeles.
In some solenostelic ferns, and in many dictyostelic ones additional vascular strands are present which do not form part of the primary vascular tube.
Sometimes a complete internal vascular cylinder, having the same structure as the primary one, and concentric with it, occurs in the pith, and others may appear, internal to the first (Matonia, Saccoloma).
In the megaphyllous forms, on the other hand, (Ferns) whose leaves are large relatively to the stem, the departure of the correspondingly large trace causes a gap (leaf-gap) in the vascular cylinder, as already described.
A considerable evolution in complexity can be traced in passing from the simplest forms of xylem and phloem found in the primary vascular tissues both among Pteridophytes and Phanerogams to these highly differentiated types.
In general structure they approach the Phanerogams with which they form collectively the Vascular Plants as contrasted with the Cellular PlantsThallophyta and Bryophyta.